In the completion of oil wells it is customary to have a succession of smaller diameter boreholes as a function of depth, each of the boreholes being lined with a tubular pipe member and each of which are successively cemented in place. The column of cement between the outer surface of the string of pipe and the borehole wall supports the pipe and prevents fluid migration.
As a matter of terminology, a string of pipe extending from the surface is commonly called a casing. The surface borehole is drilled a few thousand feet and the surface casing inserted into the borehole and cemented in a conventional manner. Thereafter, a smaller diameter bit is used to drill another section of borehole. A string of casing can be inserted into the second open borehole which extends to the surface and is cemented in place in a conventional matter. Alternatively, a liner can be suspended in the lower end of the surface casing and cemented in place. A liner is a string of pipe which is hung at the lower end of another string of pipe which is already in the borehole. After the next casing or liner is in position in the borehole, the drilling is continued to another depth and a string of pipe is suspended in the lower end of the next above string of pipe and cemented in place. The drilling operations continue thusly until the desired depth is reached.
To position and cement a liner in a string of casing, the liner is made up with the usual bottom hole equipment which includes a casing shoe, float collar and plug catchers and is connected up to the desired length. At the top of the liner is a liner hanger which is an assembly having slip elements which are normally retracted while going into the borehole and which are released downhole when setting of the liner hanger is desired. The liner hanger is lowered into the borehole by a setting tool which attaches to the liner hanger and a string of pipe attached to the setting tool. At the desired location where the casing shoe is preferably located above the bottom of the open borehole, the liner hanger is set in the next above casing by actuating the setting tool to set the slips on the liner hanger. Upon setting the liner hanger, the weight of the liner is suspended by the liner hanger on the next above casing. The setting tool is released and the liner hanger is then cemented by pumping cement through the string of pipe through the liner and into the annulus between the liner and borehole. After the cement is set up, the remaining cement in the liner is removed by drilling through the liner and destructible cement equipment at the lower end of the liner. When the open borehole reaches the projected well depth and traverses the formations to be completed, the liner includes a liner hanger and oftentimes polished bore receptacle (PBR). The polished bore receptacle attaches either above or below the liner hanger and provides a bore to receive a sealing member on a tubing string. Various types of polished bore configurations are available which include an insert polished bore receptacle in another polished bore receptacle where the insert polished bore receptacle is latched and locked into the other polished bore receptacle.
The production string of tubing which has a sealing element adapted to be slidingly and sealingly received in the polished bore receptacle, extends from the earth's surface and when the well is completed, fluids flow through the tubing string to the surface. The sealing element on the string of tubing is subjected to downhole hydraulic pressure forces and the tubing string is subject to expansion and contraction forces due to the temperature. The purpose of a sliding seal on a string of tubing in a polished bore receptacle is to permit movement of the seal and the string of tubing relative to the polished bore receptacle. However, if the proper tubing size and materials are not used, the pressures and temperatures encountered in production can cause the tubing string to become distorted in the wellbore and to be helically buckled. This occurs because the top of the tubing string is fixed to the earth's surface and the forces produced in the wellbore exceed the capabilities of the string of tubing. Thus excessive upward movement of the tubing string in the polished bore receptacle can force the tubing upwardly and distort it. This effect has been studied and it is common to calculate the amount of helical buckling which will occur and to compensate and provide the right length of tubing with proper strength capabilities in the wellbore so that the buckling effect is not so severe as to prevent the passage of tools through the tubing or to permanently corkscrew the tubing string in the casing. For background information, reference may be made to papers such as "Deep Well Completions Can Be Simple" by H. E. Lindsey, Jr., published in the Petroleum Engineer magazine in April, 1976; "Deep Well Completion Practice" by H. E. Lindsey, Jr., published by the American Institute of Mining, Metalurgical and Petroleum Engineers, Inc. in September, 1972; "Tubing Movement Forces And Stresses And Dual Flow Assembly Installations" by Kenneth S. Durham, published by the Society of Petroleum Engineers Journal in December, 1982; and, "Determining Clearance And Helicly Buckled Tubing" by H. Ed Lindsey, Jr., Charles W. McLarnan and James A. Nickel, published by World Oil in June, 1980.
In many instances, last minute changes can occur, such as a change in the completion where a more expensive tubing is not available or smaller sized tubing is desired. In such instances, it is not possible to change tubing design with respect to the polished bore receptacle.